Well as I read it Ulthok is the only one to notice he is about to cast, so if the spell needs to be interupted then I guess he's the one to do it.
Even a little halfling tripping and falling into a tall human is enough to break concentration. He might even knock him into that fireplug.
After that the rest will be more aware.
I read it that you as the only caster are the one to notice it is not a "nice" spell. All of us should be able to see him casting. Cor would assume that nothing good would come of the two of them meeting again.
For better visual referance in my campaign think of Cor as Gimli from Lord of The Rings.
As a lot of spell casting deals with hand gestures everyone can see he is casting something. Ody as Keeper has said you feel the spell is not a nice one in the energy's Allanan is drawing from.
Everyone in the room can see he is casting. However, only another mage has the ability to understand a bit of the nature of the spell being cast.
Pretty obvious the guy in robes pulling out spell components, waving his hands and speaking a weird language is either casting a spell or a lunitic.
Cor is a cleric. Rangers get cleric or druid spells. Cor had no idea what spell is being cast but history tells him that Allanon and Althena could cause big problems.
Cor wears a helm and ring that allow him to cast certain wizard spells but it is the item not Cor that powers the spells.
So, to clarify... Rayden would see a spell was going off but could not sense the harmful intent of the spell?
Like Keeper explained most spells have hand movements much like sign language. So yes easy to tell a spell is being cast much harder to tell what the spell is.
Okay, so I played Rayden's reaction correctly then lol